Thermal dealkylation of alkyl aromatic compounds employing a hydrogen donor and molecular hydrogen



United States Patent THERMAL DEALKYLATION 0F ALKYL ARO- MATIC COMPOUNDSEMPLOYING A HY- DROGEN DONOR AND MOLECULAR HY- DROGEN James C. Hill,Chesterfield, Mo., assignor to' Monsanto Company, St. Louis, Mo., acorporation of Delaware No Drawing. Filed Dec. 20, 1966, Ser. No.603,089 Int. Cl. C07c 3/58, /00

US. Cl. 260672 9 Claims ABSTRACT OF THE DISCLOSURE Alkyl aromaticcompounds are dealkylated in a thermal reaction zone in the presence ofa hydrogen donor and molecular hydrogen. The hydrogen donor is anaromatic compound that has been at least partially hydrogenated. Forexample, a pure toluene feed is demethylated at about one-half the rate(68% versus 1l14% conversions to benzene) of a toluene feed containing6% dihydrophenanthrene, in 13 seconds at 625 C., 48.6 atmospheres, andratios of H ztoluene in the range of 37, whereas an equal concentrationof phenanthrene increased the rate some 12% above the rate for puretoluene.

BACKGROUND OF THE INVENTION This application relates to assigneesco-pending applications Ser. Nos. 527,049 and 604,614.

The present invention relates to a process for the dealkylation of alkylsubstituted aromatic compounds. More particularly, the present inventionrelates to a process for the thermal hydrodealkylation of alkylsubstituted aromatic hydrocarbons.

Within recent years, thermal hydrodealkylation of alkyl substitutedaromatic compounds has become a relatively well known and commerciallyaccepted route of upgrading the value of hydrocarbon streams. By suchprocesses, the alkyl substituents of aromatic ring compounds are removedfrom the aromatic rings. These thermal processes involve subjecting thealkyl substituted aromatic compounds to elevated temperatures in theabsence of catalysts. While a number of these processes have reached thepoint of present day commercial usage, still there is'a need and adesire for a significant increase in the efliciency of suchhydrodealkylation processes.

It is an object of the present invention to provide a new and improvedprocess for the dealkylation of alkyl substituted aromatic compounds.Another object of the present invention is to provide a new and improvedprocess for the thermal hydrodealkylation of alkyl substituted aromaticcompounds. An additional object of the present invention is to provide anew and improved process for the thermal hydrodealkylation of alkylsubstituted aromatic compounds whereby substantially improved conversionrates of the alkyl substituted aromatic compounds to dealkylatedaromatic compounds are obtained. A particular object of the presentinvention is to provide a new and improved process whereby alkylsubstituted aromatic hydrocarbons may be dealkylated with increasedconversion rates to-dealkylated aromatic hydrocarbon product. Additionalobjects will become apparent from the following description of theinvention herein disclosed.

In fulfillment of these and other objects, it has been found that alkylaromatic compounds may be successfully dealkylated at high conversionsby thermal means according to the process of the present invention whichprocess comprises subjecting a feed containing alkyl substitutedaromatic compounds to a temperature of 400 to 850- C. and a pressure of100 to 2,000 p.s.i.g. in a non-catalytic, thermal reaction zone, in thepresence of molecular hydrogen in a mol ratio of hydrogen to alkylsubstituted aromatic compounds Within the range of 0.1:1 to 20:1, and inthe presence of a hydrogen donor, said hydrogen donor being a partiallyhydrogenated aromatic compound. The process of the present inventionprovides a method for the thermal dealkylation of alkyl aromaticcompounds at somewhat lower temperatures than those conventional toother processes. Further, significantly higher conversion rates areobtained by the present process than are obtained through the use ofsimilar but different dealkylation processes. Another advantage of theprocess of the present invention is that alkyl substituents generallyare severed from the alkyl aromatic compound as one molecule rather thanas several lower molecular weight molecules, thereby preserving thealkyl molecule and reducing the hydrogen consumption.

To further describe and to specifically illustrate the presentinvention, the following examples are presented. These examples are notto be'construed in any manner as limiting the conditions, application orobjects of the present invention.

EXAMPLE I A series of nine runs were made using toluene as the alkylsubstituted aromatic compound. Three of these runs were made inaccordance with the present invention with both molecular hydrogen and9, lO-dihydrophenanthrene, as a hydrogen donor, being added to thetoluene feed. In three other of the runs, the hydrodealkylation reactionwas carried out in the presence of molecular hydrogen only, no hydrogendonor being present. The remaining three runs were caried out in thepresence of molecular hydrogen and phenanthrene, the purpose of thesethree runs being to illustrate that the advantages derived from adding ahydrogen donor such as the 9,10-dihydrophenanthrene are not merelydiluent effects. In all nine runs, the toluene and hydrogen togetherwith the 9,10-dihydrophenanthrene or phenanthrene, if any, werecontinuously and concurrently passed through a reaction chamber havingan inside diameter varying from inch at the entrance to inch at the exitand having a length of 7% inches. Temperature within the recationchamber was 625 C., the residence time of the reactants therein 13seconds, and the pressure was 700 p.s.i.g. The table below summarizesthe flow rate of the various materials going into the reaction chamber,the molecular hydrogen to toluene ratio, and the conversion of toleueneto benzene for each of the nine runs.

Percent H2/ conver- Phenantoluene sion threue From the above table, thepoorest of Runs 1, 2 and 3 carried out in accordance With the presentinvention, Run No. 1 represents a 30.6 percent increase in conversionover the best of Runs 4 through 9 which were not carried out inaccordance wiht the process of the present invention. The best of theruns carried out in accordance with the present invention, Run 3,represents a 70.6 percent increase in conversion as compared to Run 9which was not carried out in accordance with the process of the presentinvention. Comparison of Run 1 with Runs 4 and 7 which had hydrogen totoluene ratios similar to Run 1 further illustrates the advantages ofthe process of the present invention.

3 EXAMPLE II Runs 1 through 3 of Example I are substantially repeatedwith the .exeception that Tetralin is used as the hydrogen donor ratherthan 9, IO-dihydrophenanthrene. In each of the runs a good conversion oftoluene to benzene is obtained.

EXAMPLE III Runs 1 through 3 of Example I above are again substantiallyrepeated with the exception that the alkyl substituted aromatic compoundis l-methylnaphthalene and the temperature within the reaction chamberis approximately 650 C. In each of the runs a good conversion of thel-methylnaphthalene to naphthalene is obtained.

EXAMPLE IV Run 1 of Example I is again substantially repeated with theexeception that the alkyl substituted aromatic compound is ethylbenzene.A gOOd conversion of the ethylbenzene to benzene and ethane is obtained.

Suitable feed stock The alkyl substituted aromatic compounds which maybe dealkylated in accordance with the hydrodealkylation process of thepresent invention include practically any aromatic compound containingalkyl substituents of one or more carbon atoms per alkyl substituent.These aromatic compounds may be mono-nuclear or poly-nuclear and maycontain one or more alkyl substituents. For example, the feeds to theprocess of the present invention may be mono-, di-, tri, or tetra-alkylsubstituted aromatic hydrocarbons, such as dimethyl benzenes, trimethylben zenes, dimethyl naphthalenes, tirmethyl naphthalenes, tetramethylnaphthalenes, diethyl benzenes, toluene, ethyl benzene, methylnaphthalene, diethyl naphthalenes, methyl phenanthrene, dimethylanthracenes, dimethyl pyrenes, tetraethyl phenanthrenes, dimethylchrysenes, tetraethyl pyrenes, trimethyl anthracenes, diethyldimethylphenanthrenes, methylethyl benzene, methylethyl naphthalene, and thelike. The alkyl substituents of the aromatic compounds which may bedealkylated in accordance with the present process may be eitherstraightchain or branched-chain alkyl substituents and may contan 1 to20 carbon atoms and higher. The process of the present invention isequally applicable to the dealkylation of alkyl benzenes and/ or alkylnaphthalenes and/ or alkyl phenanthrenes and/ or alkyl anthracenes and/or alkyl pyrenes and/or alkyl chrysenes and the like. In addition, suchalkyl aromatic compounds as acenaphthenes, acenaphthenes, alkylfiuorenes, alkyl indans, alkyl indenes, and the like may be dealkylatedin accordance with the present process. In addition, the presentinvention finds application in the dealkylation of alkyl aromaticcompound containing substituents other than alkyl groups. For example,the alkyl aromatic compounds may contain hydroxyl, alkoXy,alkoxycarbonyl, halogen, sulfide, sulfate, nitrate, amino, nitrile,nitro and other such radicals as substituents in addition to alkylsubstituents. Also, the aromatic compound may contain elements otherthan carbon in the aromatic nucleus. For example, the present inventionmay be utilized in the dealkylation of alkyl pyridines, alkyl pyrans,alkyl furans, and alkyl substituted thiophenes. Also, the presentinvention is useful in the dealkylation of complex mixtures of the abovealkyl aromatic compounds as well as the pure compounds. In the preferredpractice of the process of the present invention, the alkyl aromaticcompounds are alkyl aromatic hydrocarbons having no greater than twocarbon atoms in the alkyl substituents.

The hydrogen donors which are used in the process of the presentinvention are aromatic hydrocarbons which have been at least partiallyhydrogenated. Generally, these hydrocarbons are di-nuclear orpoly-nuclear aromatics having one or more of the nuclei partially ortotally saturated. Several non-limiting examples of such compounds areTetralin, dihydronaphthalenes, diand tetra-hydroalkylnaphthalenes,dihydrophenanthrenes, tetrahydrophenanthrenes, octahydrophenanthrenes,tetrahydrophenylnaphthalenes, dihydrochrysenes, tetrahydrochrysenes,octahydrochrysenes, tetrahydropyrenes, octahydropyrenes,tetrahydrofluorenthenes, octahydrofiuorenthenes, and the like. Aparticularly useful group of these hydrogen donor compounds are thehydrophenanthrenes and hydronaphthalenes. The source of the hydrogendonor used in carrying out the process of the present invention isimmaterial. These hydrogen donors may be obtained by separatinghydrocarbon fractions to obtain the aromatics which have been at leastpartially hydrogenated or may be obtained by hydrogenating specificaromatic hydrocarbons by conventional hydrogenation means. The presentinvention is not, however, to be limited to any particular source ormethod for obtaining the hydrogen donors.

The mol ratio of the hydrogen donor to the alkyl aromatic compounds inthe thermal delakylation zone is most often within the range of from0.01:1 to 10:1. However, in a preferred method of practicing the processof the present invention wherein the herein below defined preferredamounts of molecular hydrogen are used, the mol ratio of hydrogen donorto alkyl aromatic compounds in the dealkylation zone is usually withinthe range of from about 0.05:1 to 4:1. The molecular hydrogen which isused with the hydrogen donor is generally present in a molar ratio ofthe alkyl aromatic compounds within the range of 0.1:1 to 20:1.Preferably, however, the mol ratio of hydrogen to alkyl aromaticcompound is within the range of 2:1 to 10:1.

The temperatures at which the present process is most often operatedgenerally are within the range of from approximately 400 to 850 C. Attemperatures below the lower temperature, the desired dealkylationreaction falls to a rate too low for practical utilization. Attemperatures above 850 C., the aromatic nucleus of the various alkylaromatic compounds as well as the hydrogen donors will begin to ruptureexcessively causing loss of aromatic product and severe carbon formationand coating of the reaction chamber. A particular preferred range oftemperatures for operating the present invention are those within therange of from approximately 550 to 750 C.

Pressures at which the hydrodealkylation process of the presentinvention is usually operated are most often within the range of fromapproximately to 2,000 p.s.i.g. Preferably, however, the pressure atwhich the present thermal hydrodealkylation process is operated will bewithin the range of from about 500 to 1500 p.s.i.g.

The residence time of the reactants within the thermal hydrodealkylationzone most often is within the range of from about 1 second to about 60minutes and longer. If longer residence times are used, there is alikelihood of destruction of a part of the feed material, products andhydrogen donor compounds resulting in poor efficiency and in theformation of undesirable carbon and coke. At lower residence times, theconversions and yields are far too low for practical application of thepresent invention. Residence time, of course, is dependent to a largeextent upon the temperature and to a lesser extent upon the othervariables of the process. If higher temperatures are used, lowerresidence times are required and vice versa. The present invention maybe operated as either a batch operation or as a continuous flow system.Temperatures are usually somewhat lower in the former, thus requiringlonger residence time. Conversely, in the continuous flow systems,higher temperatures with shorter residence times are used. In batchoperations, the residence time is usually within the range of 10 secondsto 60 minutes while the preferred residence times for continuous flowoperations are usually within the range of from about 1 second to 20minutes, preferably 1 to 100 seconds. From a practical standpoint, it isgenerally preferred that the present process be operated as a continuousflow system.

The particular individual equipment used in carrying out the process ofthe present invention is not critical as long as it conforms to goodengineering principles and will allow eflicient operation under theconditions set forth herein.

What is claimed is:

1. In a process for the hydrodealkylation of alkyl substituted aromatichydrocarbons by subjecting alkyl substituted aromatic hydrocarbons to atemperature of 400- 850 C. and a pressure of 100-2000 p.s.i.g. in anon-catalytic, thermal reaction zone in the presence of molecularhydrogen in a mol ratio of molecular hydrogen to alkyl substitutedaromatic hydrocarbons of 0.1:1 to 20:1, the improvement which comprisescarrying out said subjecting in the presence of an aromatic hydrocarbonhydrogen donor which is a phenanthrene hydrocarbon that has been atleast partially hydrogenated in a mol ratio of hydrogen donor to alkylsubstituted aromatic hydrocarbons of 0.05:1 to 4:1.

2. The process of claim 1 wherein the residence time of the reactantswithin the non-catalytic, thermal reaction zone is within the range of 1second to 60 minutes.

3. The process of claim 1 wherein the temperature is within the range of550 to 750 C.

4. The process of claim 1 wherein the pressure is within the range of500 to 1500 p.s.i.g.

5. In a process for the hydrodealkylation of alkyl substituted aromatichydrocarbons by subjecting a feed stock consisting essentially of alkylaromatic hydrocarbons to a temperature of 400-850" C. and a pressure of100-2000 p.s.i.g. in a non-catalytic, thermal reaction zone in thepresence of molecular hydrogen in a mol ratio of molecular hydrogen toalkyl substituted aromatic hydrocarbons of 2:1 to :1, the improvementwhich comprises carrying out said subjecting on a mixture ofdihydrophenanthrene with the alkyl aromatic hydrocarbon in a mol ratioof dihydrophenanthrene to alkyl aromatic hydrocarbon in the range of0.05:1 to 4:1.

6. The process of claim 1 wherein the alkyl substituted aromatichydrocarbons are alkyl aromatic hydrocarbons selected from the groupconsisting of alkyl benzenes, alkyl References Cited UNITED STATESPATENTS 2,381,522 8/1945 Stewart 196-50 2,929,775 3/1960 Aristolf et a1208-133 2,994,726 8/1961 Hodgson et al 260-668 3,102,151 8/1963 Haldemanet al. 260-672 3,145,238 8/1964 Kestner 260-672 3,193,595 7/1965 Kentonet al. 260-672 3,198,846 8/1965 Kelso 260-672 3,256,357 6/1966 Baumannet a1 260-672 3,284,527 11/1966 Gill et a1 260-672 3,288,873 11/1966Moll 260-672 3,288,875 11/ 1966 Payne et al. 260-672 3,296,323 1/1967Myers et a1 260-672 3,177,262 4/1965 Calkins 260-672 3,320,332 5/1967Schneider 260-668 OTHER REFERENCES Curran et al.: Mechanism of HydrogenTransfer,

US. Cl. X.R. 260-668, 675

